1. Field of the Invention
This invention relates to an improvement in an semiconductor laser device having a strained quantum well structure.
2. Prior Art
FIG. 5 of the accompanying drawings illustrates a known strained quantum well type semiconductor laser device.
Referring to FIG. 5, the semiconductor laser device comprises an n-InP buffer layer 2 and a nondoped GaInAsP light confining layer 3 are sequentially laid on an n-InP substrate 1 and then a nondoped InAs.sub.y P.sub.1-y strained quantum well layer 4a, a nondoped GaInAsP barrier layer 4b and a nondoped InAs.sub.y P.sub.1-y quantum well layer 4c are sequentially laid thereon to form a light emitting active layer 4. Thereafter, a nondoped GaInAsP light confining layer 5, a p-InP clad layer 6 and a p.sup.+ -GaInAs contact layer 7 are sequentially laid thereon to form a multilayer structure.
Of the layers, each of the light confining layer 3, the barrier layer 4b and the light confining layer 5 may be made of nondoped GaInAsP and have a wavelength specific to its composition which is equal to 1.1 .mu.m, while the strained quantum well layer 4a may be made of nondoped InAs.sub.y P.sub.1-y and have a film thickness of 80 .ANG., assuming its composition ratio y to be 0.55, and the semiconductor laser device may have a resonator length of 900 .mu.m.
A strained quantum well type semiconductor laser device as illustrated in FIG. 5 and having the above specified values provides a low threshold current density of approximately 450 .ANG./cm.sup.2.
As is well known, a well structure is formed for an energy band in a semiconductor laser device having an above described configuration, utilizing the compositional difference of the strained quantum well layer 4a, the light confining layers 3 and 5 and the barrier layer 4b.
If, for instance, the composition ratio y of the strained quantum well layer 4a comprising V-group elements is 0.55, a compression stress is applied to the strained quantum well layer 4a due to the difference of lattice constant to deform the layer by 1.77%.
In general, a strained quantum well layer of the type under consideration preferably has a large film thickness and/or a multilayer structure comprising a relatively large number of component layers from the viewpoint of reducing its oscillation threshold current level and raising its high frequency performance. However, a large film thickness and/or a multilayer structure of a strained quantum well layer can give rise to crystal dislocations if the strained quantum well layer is deformed to the above described extent.
It is known from experiments that a semiconductor laser device has a critical film thickness (total film thickness if it has a multilayer structure) of approximately 300.ANG..
Thus, the semiconductor laser device 4a may appropriately have a multilayer structure comprising two to three component layers from 300 .ANG. (critical film thickness)/80 .ANG. (thickness of each component film).
A conventional strained quantum well layer type semiconductor laser device typically shown in FIG. 5 may have a strained quantum well layer comprising only few component layers to raise the carrier density that each of the component layers of the strained quantum well layer bears, meaning that an enhanced carrier density is needed for oscillation.
This is why a conventional strained quantum well layer type semiconductor laser device is less effective than a nonstrained quantum well layer type semiconductor laser device in terms of oscillation threshold current, oscillation efficiency at high temperature range and high frequency performance.
In view of these technological problems, it is therefore an object of the present invention to provide an improved strained quantum well layer semiconductor laser device in terms of low threshold current, high temperature operation and high frequency performance.